As one of standard signal generators, there is a frequency synthesizer to which PLL (Phase Locked Loop) is applied. As shown in FIG. 14, the frequency synthesizer N-divides a frequency of a voltage controlled oscillator 202 by a frequency divider 202 to input a frequency divider output to one of input ends of a phase comparator 203, and in addition, M-divides an oscillation output of, for example, a quartz-crystal oscillator 204, which is a reference signal generator, by a frequency divider 200 to input a frequency divider output to the other input end of the phase comparator 203, and feeds a comparison signal between these outputs back to the voltage controlled oscillator 201 via a loop filter 205, thereby forming a PLL (for example, patent document 1). When the PLL is locked, a relation between a frequency fvco of the oscillation output of the voltage controlled oscillator 201 and a frequency f0 of the oscillation output of the quartz-crystal oscillator 204 is fvco/N=f0/M, and therefore, fvco=(N/M)f0. The frequency divider 202 is formed by a programmable counter and the frequency divider ratio N thereof can be externally set by digital data, enabling the free setting of the frequency fvco.
The application of the frequency synthesizer includes its use as, for example, a local oscillating part in a mobile station. Specifically, since a base station allocates a predetermined frequency band to a mobile station, the mobile station side needs to generate an oscillation output in the allocated frequency band, which necessitates the local oscillating part to have a function of capable of adjusting the frequency. The frequency synthesizer is also used as a test signal source of a radio communication device, a broadcasting device, and the like.
When the frequency synthesizer is thus applied in, for example, a communication field, its noise has to be small in order to avoid the interference with other channels, and it is desirably capable of setting the frequency as finely as possible because radio waves have been congested. Increasing the aforesaid frequency divider ratio N enables the fine setting of the frequency, but an excessive increase in the frequency divider ratio N causes an increase in noise due to an elongated delay occurring in the loop, and actually about 1000 is an upper limit of N.
Therefore, in order to design a frequency synthesizer capable of adjusting the frequency, for example, in about 1000 MHz range for convenience sake, in 1 Hz increments, the device in FIG. 14 has to be multi-staged. Specifically, assuming that the upper limit of N is 1000, it is possible to produce a frequency synthesizer capable of adjusting the frequency within a 1 MHz to 1000 MHz range in 1 MHz increments, by setting the frequency (f0/M) of the reference signal input to the phase comparator 203 to 1 MHz. Likewise, it is possible to produce a frequency synthesizer capable of setting the frequency within a 1 kHz to 1 MHz range in 1 kHz increments by setting the frequency of the reference signal to 1 kHz, and similarly, it is possible to produce a frequency synthesizer capable of setting the frequency within a 1 Hz to 1 kHz range in 1 Hz increments, by setting the frequency of the reference signal to 1 Hz. Then, by synthesizing these frequency synthesizers in stages, it is possible to obtain a frequency synthesizer capable of setting the frequency up to 1000 Mherz in 1 Hz increments.
However, the above method requires the formation of PLLs for respective synthesizing circuits synthesizing the frequencies, and thus has a problem of complicating the circuit configuration and increasing the number of components, leading to great noise.
Therefore, the present inventor has developed a frequency synthesizer of a new type capable of finely setting the frequency over a wide band by adopting a new structure having a totally different principle from that of a conventional frequency synthesizer (for example, patent document 2), and as its element technology, has studied various circuit configurations enabling stable pull-in of the frequency even when there are product variations in voltage controlled oscillating parts, a temperature characteristic change, and the like.